Finally I took my Maverick and did some probing. I calibrated the probes in ice water before the test and all were withing +/- 2° of 32° so I felt good enough about them. I placed them unscientifically in the grill. One in the middle of the bottom rack, two on the right and left of the middle rack, and one in the middle of the top rack. (no photo, sorry) I jammed them all through bread so they were not touching any metal. Below is a photo after 30 minutes of the grill stabilizing at 250°
As you can see the grill setting and RTD both think they are at 250°. The Maverick showed a 5° temp swing. I find this temp swing to be statistically irrelevant because it is so minor that I did not even record which probe was what temp. Between this test and the bread test I feel pretty confident in the BFG's even heating at reasonable temperatures.
(I don't know why this one is sideways, or how to fix it)